Electrical motive power systems are used to drive many types of mobile machines. Diesel-electric locomotives, electric cars and material-handling cranes are but a few examples of machines driven by electric motors.
For the person engineering the motive power system, one type of material-handling crane presents an unusual problem. A so-called portal crane, often referred to as a gantry crane, is shaped like an inverted "U" and the lower ends of its two spaced support legs are attached to wheeled "trucks" which ride atop spaced rails. Each leg and truck component is driven by a separate motor and each moves at a particular speed. And while it is intended that both legs move simultaneously and at the same speed, this is not always the case.
The electric motors driving the legs are mechanically separated. That is, there is no mechanical connection, e.g., by a line shaft or the like, between the motors. Rather, such motors are free (within certain limits) to run at different speeds and at different rates of acceleration. Under certain crane operating conditions, the fact of "mechanical disconnectedness" results in undesirable crane skewing.
The legs support a horizontal "bridge" positioned well above the ground or floor and on which a loadhandling "trolley" moves. The crane is arranged so that the trolley can move along the entire distance between the legs. Often, the bridge extends beyond at least one of the legs and the trolley can also move "outboard" of that leg, i.e., along such extended bridge length.
In a typical portal crane, one leg (often referred to as a "fixed leg") is rigidly attached to the bridge while the other leg, often referred to as the "hinged leg," has limited freedom of motion to pivot about an axis along the bridge and in a plane coincident with the hinged leg. Because of the rigid attachment of the fixed leg and the bridge, that leg can (and for reasons explained below, sometimes does) "pull along" the bridge and the other leg. This tends to skew the crane.
Skewing often arises because the legs are unevenly loaded--one leg tends to lead the other. If the trolley and its load are very near, over or even outboard of one leg, that leg will be more heavily loaded and will tend to lag and move more slowly. And portal cranes are often used outdoors. "Wind loading," i e. the force resulting from wind blowing against the trolley, cab and load, can cause skewing if the trolley is nearer one leg than the other. And the crane control house, that enclosure in which electrical equipment is housed, may be located nearer one leg than the other. Such house is exposed to the wind. Additionally, it should be appreciated that wind loading need not only tend to slow one leg of the crane. Eccentric crane loading and wind can tend to accelerate and cause one leg to lead, depending upon wind direction and the direction of crane travel.
While portal cranes are designed to accept and withstand a modest amount of skewing, almost any amount of skewing tends to put additional stress on parts of the crane. And excessive skewing can stress such parts unduly and cause premature failure.
One approach that helps prevent undue skewing is to simply manipulate the master switch less "aggressively" so that rates of acceleration and speeds do not become badly mismatched. However, the utility of the crane is thereby impaired; it simply does not operate at the increased "duty cycle" that, in view of the invention, is now possible.
("Duty cycle" may be explained in terms of the time required for a machine to make one load-handling round trip. The longer such required time, the lower the duty cycle. Clearly, duty cycle is a measure of machine productivity and the ability of the crane owner to attain a satisfactory return on the substantial investment.)
An approach that has been used to prevent undue skewing involved a so-called "bang bang" control, an operating principle of which was to momentarily shut off the electric motor driving the leading leg of the crane. But this approach was not effective during the initial 10-15 seconds over which the crane was accelerating from, say, a standstill. The main control system did not respond to the "shut off" signal until after initial crane acceleration. But by then, structural damage may have occurred.
An improved system and method overcoming some of the problems and shortcomings of the prior control systems would be an important advance in the art.